Introducing RR

I’m pleased to introduce a new Test Double (or mock) framework named RR, which is short for Double Ruby.

Why a Double framework and not a Mock framework?

A mock is a type of test double. Since RR supports mocks, stubs, and proxies, it makes sense to refer to RR as a double framework. The proxy is a new usage pattern that I will introduce later in this article, and in more detail in future articles.

Unfortunately, the terminology over doubles has been contradictory depending on the framework. RR’s terminology tries to be as faithful as possible to Gerald Meszaros’ definition of test doubles. You can read more about test doubles in XUnit Test Patterns and Martin Fowler’s article, Mocks aren’t Stubs. Regretfully, this does mean that RR will have slightly different terminology than other double frameworks.

How does RR compare to other Mock frameworks?

Most double frameworks focus mainly on mocks (hence the categorization “mock framework”). RR’s focus is on enabling more double test patterns in a terse and readable syntax.

RR also does not have dedicated mock objects. It primarily uses the technique called ‘double injection’. Names that other frameworks use are ‘stub injection’, ‘mock object injection’, ‘partial mocking’, or ‘stubbing’. The term I’ll use for this is a double injection, since one or many doubles are being injected into an object’s method.

I’ll use trivial Rails examples to highlight the syntactical differences between RR, Mocha, Rspec’s mocks, and Flexmock. They may or may not be appropriate situations for mocks. The right situations for mocks is an entirely different discussion.

If there is better way to do any of the examples, please post a comment and I will gladly replace it.

Mocks

Here are the ways to mock the User.find method. The expectation is the User class object will receive a call to #find with the argument ’99’ once and will return the object represented by the variable user.

RR

mock(User).find('99') { user }

Mocha

User.expects(:find).with('99').returns(user)

spec/mocks

User.should_receive(:find).with('99').and_return(user)

Flexmock

flexstub(User).should_receive(:find).with('99').and_return(user).once

Stubs

Here are the ways to stub the User.find method. When the User class object receives a call to find with the argument ’99’ it will return user1. When User receives find with any other arg, it returns user2.

Flexmock

Proxy

A proxy used with a mock or stub causes the real method to be called. Expectations can be placed on the invocation and the return value can be intercepted. The main rationales are test clarity and you can ensure that the methods are being called correctly, even after you refactor your code. I will delve more into proxies and their usage patterns in my next article.

Mock Proxy

The following examples set an expectation that User.find(’99’) will be called once. The actual user is returned.

RR

mock.proxy(User).find('99')

Mocha

You cannot implement this in Mocha. You can do an approximation in this situation however. This technique is not always the solution you need, though.

Flexmock

instance_of

instance_of is method sugar than allows you to mock or stub instances of a particular class. The following examples mock instances of User to expect valid? with no arguments to be called once and return false.

Flexmock

More to come

This concludes the introduction to RR. RR enables some techniques, like proxying, that will make your tests clearer and less brittle. In the next article I will describe into patterns and techniques that will make mocks a more feasible tool for more situations.

17 Comments

Carl says:

The introduction of different mocking/stubbing strategies that RR implements is quite refreshing and I can certainly appreciate its terseness. I’m looking forward to your next post and am particularly interested in how RR can be used to stub out instances of unimplemented classes.

This looks great. Without having known the terminology, I’ve been wishing that Mocha could setup an expectation that still passed the method call back to the original object. Proxying will definitely come in handy.

Yeah, the all in one line proxying would be handy, but in Mocha specs I’ve just added one more line aliasing a method before stubbing it, and then returning the call on that, as needed (where the proposed preset reference solution wasn’t appropriate). It might not be quite as flexible as true proxying, but since I’m mocking everything out anyways, the fixed values are fine and it’s worked well enough when I’ve needed.

Sergey, unfortunately I’m not able to reproduce this issue on Rails 2.1.0. Can you paste your test on http://pastie.caboo.se?

Thanks,
Brian

June 19, 2008 at 6:22 am

James says:

How do I call the actual method (or any method for that matter) on the class I’m trying to stub/mock/proxy?

Example:

`
class MyObj
def self.meth1; end
def self.meth2; end
end

mock.proxy(MyObj).meth1 { meth2 }
`

Also what if I want to proxy a method but so that the mocked proxy is called FIRST, which would allow me to intercept the call before it’s being made. The current mock.proxy() seems to call the original method first and then the proxied code. If I’d want the proxy to be called first, and perhaps decide in the proxy block if the real method should be called or not..how do I do that?

Thanks..

September 25, 2008 at 1:11 pm

Brian Takita says:

@James: Both use cases are interesting things that I have yet not considered.

The block is not instance_exec’d, so:

mock.proxy(MyObj).meth1 { meth2 }

would not work.

Here is a way to solve it.

mock.proxy(MyObj).meth1 { MyObj.meth2 }

Perhaps it should be instance_exec’d. I’ll play around with it.

Regarding the method interception, you could do the following:

original_meth1 = MyObj.method(:meth1)
mock(MyObj).meth1 do
# do something
original_meth1.call
end

I’d be willing to add a more explicit hook if a nice api can be found.
Maybe something like:

mock.proxy(MyObj).meth1.before {# do something}.after {# This is the return value}

Such a change would be additive, so:

mock.proxy(MyObj).meth1 {# This is the return value}

would still work.

September 30, 2008 at 7:48 pm

Jaime says:

Great library thanks!

Have a bit of a problem, when the unit test in question defines the “teardown” method, none of the RR related assertions work. Is this normal?